The building codes and fire safety codes of many political jurisdictions in the United States, as well as standard contract language provided as a practical necessity in many construction bidding situations, and many fire insurance policy terms require that the sprinkler units of the automatic fire sprinkler system of the building in question must be ones which are of a design which is certified as having passed the standard test of a specified independent testing laboratory. Although at present other testing laboratories are acting to attempt to obtain some significant proportion of the certification business, historically, and to the present, almost all of the certification has been done either by Underwriters Laboratories, Inc. or Factory Mutual. In any event the testing requirements and procedures of the different accepted testing organizations have many common features.
For automatic fire sprinkler units a typical testing requirement is that the unit have a resistance to structural failure from overload that is a stated multiple of its operational sealing pressure. Multiples of in the range of from seven to one up to ten to one are common. Accordingly, there is some incentive for the manufacturer to design automatic fire sprinkler units having low to moderate operational sealing pressures so as to avoid the need for the truly excessive mechanical strength that would otherwise be needed solely so the units would be able to pass the conventional certification tests. For operational sealing, the units must be capable of static loading, perhaps over a several-year time span punctuated by pressure fluctuations caused by water hammer, temperature variation and long term effects of seal galling, material creep.
A good design of automatic fire sprinkler should be economical to produce, easy to assemble, and not prone to being damaged in shipping and installation, and especially to not be prone to hidden damage that does not become manifest until the system incorporating the installed units is pressurized and placed in operation.
It has long been the practice for the manufacturers of automatic fire sprinkler units to provide machined, sharp, knife-like edges on the sprinkler body or frame as a seat for the seal. This is to provide a high unit stress load on the soft seal material with the lowest possible load applied to the eutectic strut or other heat-responsive actuator. Such light-duty stress loading of the eutectic strut or the like is a requirement placed on the manufacturer by fire insurance companies and underwriters. Light stress loads on both the seal material and the eutectic strut are highly desirable, as this reduces the fatigue rate of these components caused by stress corrosion and material creep over the long term life of the sprinkler.
Inherent with knife-edged seats are three disadvantages that lead to continuing problems for the manufacturer. First, and perhaps the most important, is the fact that the unit stress load at the knife edge tends to decrease with increasing internal fluid pressure.
Secondly, very little overload can be applied to the knife edge surface without danger of cutting through the soft seal material.
And finally, frequent tool sharpening is required in the manufacturing process to prevent tool chatter from causing defective seats.
Conventionally, in order to correct these problems, it has been necessary to apply a greater stress load than desirable to both the seal and eutectic strut or the like to insure against leakage due to pressure surges caused by water hammer and the like. Such increased seal and strut loads remain for the long life of the sprinkler although the need for increased loads may indeed be present for a very small fraction of the sprinkler life.
Also, knife-edged seats become a serious problem if the sprinkler frame suffers a blow of some sort during the handling and installation. The seal may become damaged in this process and not detected until water damage has occurred at its installed location.
Although these problems associated with knife-edged seals are well-known in the trade, the advantage provided by the small area (high unit load) seal contact in providing for the minimum strut loads has prolonged the use of this seal arrangement.
To fix these overload/underload problems, others have chosen to place springlike washers between the strut and seal surfaces, which reduces but does not eliminate these recurring problems.